Let's Make Robots!

Questions about my hacked RC car arduino robot

Greetings everyone. I have an old RC car with 6V accumulator. Can I power my arduino board + the l293d motor driver from this accu without using any voltage regualtors? I read that arduino and 3 of the 4 pins on the l293d motor driver require 5V. 

The voltage requlator I have requires 2.5V more ie 7.5V, but the accu is 6V. More stupid questions will follow :D

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Yeah, I was ready to suggest you try another battery because I felt yours may not supply enough, but 330mA I wouldn't have thought eighter. At least you've learned some things...

You can stack your h-bridge chips like Elijiah did, or you can buy an high power motor controller like this one (overkill, perhaps) OddBot designed, or you can build you own like any listed in this h-bridge matrix.

I was spending last two days fighting with an IR range finder G2D120X wich was a nightmare to find out the voltage/distance formula but excel proved to be a saver here. Nevetheless more questions for you, as always =)

1. The L293D gives me trouble and headache again. 

For exampe we have INPUT1 and INPUT2 set to LOW. The OUTPUT1 and OUTPUT2 should have 0V but give 0.5V / -0.5V.

Now I set INPUT1=HIGH and INPUT2 = LOW. OUTPUT2 should be our GND but it gives 0.8V now (sic!). If I measure OUTPUT1 I should get VSS voltage which is 7.8V. But here is the thing. As soon as I set INPUT1=HIGH Vss drops to 7.2V!!! OK, so the OUTPUT1 should have 7.2V then. But no, real voltage is then 7.2V-0.8V=6.4V. The 0.8V is on the OUTPUT2 that should act as GND with 0V. Iam loosing 7.8V-6.4V = 1.4V and I dont know why?

2. Also when motors start to spin the voltage on the OUTPUT pins drops instantly to 5.8V raising to 6.4V over 5-10 seconds. Firstly shouldn't 470uF caps avoid excatly this behaviour? I have 2 470uF caps before and after the power regulator and additionally 1000uF on the VSS pin of the L293. And secondly is there a way to not let the voltage fall at the start?

3. If I dont have the datasheet for the motor how do I find out the optimal maximal current it requires to operate at the full power? Do I just measure the current on the motor while the car is driving?

4. When should I place a screw/heatsink in to 7805 power regulator? I mean up to what current?

5. I dont know how many arduino users are here but I do have one thing I dont get. Many videos and tutorials use USB cable to power the arduino. If I do that, I get only 4.07V on the Vin pin and 4.7V on any digital pin if set to HIGH. This is a serious problem, because my IR range finder needs 4.5 to 5.5 V to operate. The recommended voltage on the arduino is 7-12V with 6V being minimum. Are all the people underpower their arduino and their sensors etc? Again, the Vin pin gives only 4.07V which is unacceptable.

Because of the above I hate to say the car doesnt have enough power to spin around like it originally did with the remote. Really sad :( And please dont ban me for so many questions :p

1. Why? because you lose some voltage over the diodes inside the H bridge.

2. Your motors draw a LOT of current at start, because of that, the voltage drops and slowly raises back up. Why slowly? Because the capacitors need to recharge and the motors still draw more current until they settle at rolling speed. Solution? Add more 1000uF capacitors at the Vss pin (the pin of the H bridge where the motor power connects). More that I think about it, you may have a current limiting spot that prevents the motors to get as much as needed. It may be as simple as a bad connection or a too thin wire...

3. Power the motor directly from the battery and hold still the axle. That should be the max current the motor can draw.

4. When it's hot to the touch.

5. When you power your Arduino from the USB, the 5V from the USB goes directly to the regulated side of the voltage regulator. The fact that you can measure 4 V on the Vin side of the voltage regulator shows that there is a reverse voltage leak with a drop over the voltage regulator itself. You can't power anything from that voltage, as the current is very low. To minimize the voltage drop when the IR sensor is working, add a big capacitor on the regulated voltage of your board and don't connect anything to the Vin pin (the motors for instance).

First of all thank you for your extended reply, really helpful. Follow up questions:

1. Aha, so that 0.5V raising to 0.8V is exactly the voltage drop of the diode? Makes sence. But here is the thing regarding the VSS. The VSS voltage is directly connected to the unregulated 7.8V. As soon as the motor starts spinning the overall supplied voltage which is also VSS drops to 4V but raises only up to 7.4V with in a minute then my hand died holding the car. I removed all the big caps to make more excat measurment.  The unregulated voltage comes from a power supply which should have enough power I suppose. So why the drop and is it OK ?

2. Regarding the caps. Can I just add unlimited amount of them (and boy I woud) ?  Is there a drawback? The wiring seems solid though coming through a breadboard but II will test it out more thoroughly.

5. Since I upload a lot I like to have the USB cable always connected to the board. Be cause of the low voltage on the Vin pin I additionally connected the Vin pin to the unregulated 7.8V. Can I have it be coneccted both ways or does the Vin pin acts like an "output" while powering from the USB? 

Thx again for you help.


1. Because now you know your power supply can offer way too little current for those hungry motors, that's the reason for such a big voltage drop. Change the power supply and probably it will all work fine.

2. You can add as many caps in parallel as you like, but look at the voltage rating, should be at least double the battery voltage. Also, the breadboard contacts may be a current limiting factor.

5. The USB should power just the Arduino and what ever you have plugged on the 5V or 3.3V pins. Vin pin should not have any power on it, but perhaps it may have some leakage from the voltage regulator. 

I will order two 6,800uF caps to connect in parallel. That should be fun :P

Just a tip: You have named this topic "Beginner starting out - need help"

That does not provide a good headline for the next person with the same problem. 50% of all posts could have that headline :)

It would be cool if you would rename it to something like "Power my arduino board + the l293d motor driver from RC accu", or something like that. Thanks :)

Ok I kinda fixed everything for now and everything works. Things that helped:

1. Replacing battery with a power supply with higher voltage. Battery was 6V, power supply 7,4V

2. Replacing 470uF cap with a 1000 big one.

3. Adding 1-2 seconds stop time to change direction

Regarding the capasitors. They are of a great help, but can I just add more caps in parallel to help the motors? I will do more tests now, but theoretically more caps should help no ?

Now another topic. I wanted to buy a RF link to be able to control my robot, but yesterday I got an idea that if you have a working RC car with a remote you can just hook up the cables that supposed to be going to the motors into digital pins of an arduino that then controls the motor. My board was working while still in the car, but after some soldering the board behaves wiredly. Look at the foto. There are 8 black thingy with a hole in that are getting very hot to the point I burn my finger. Do you know what those are? I think Iam having a short circuit there but cant figure out why. 


The small capacitors on the motor filter noise.  If you increase the size then you change the characteristics of the filter.

There are 8 black thingy with a hole in that are getting very hot to the point I burn my finger. Do you know what those are?

Power transistors, they get hot with excessive current and no heat sinks